It was written by disgruntled reformist deputies who've been barred from standing in the general election.

Criticising the supreme leader is regarded as a serious offence.

The two newspapers ordered to close down with immediate effect were Yas-e-no, the daily of the biggest reform party, and Shargh, another reformist paper which publishes the writings of many liberal thinkers.

They were the only publications that dared to defy an order from Iran's Supreme National Security Council not to publish the text of the letter.

However, they did leave out some of the most sensitive parts, says BBC Tehran correspondent Jim Muir.

Freedom of speech?

A low turn-out is expected at Friday's general election

The MPs and the two newspapers may have been emboldened by remarks made by the Ayatollah himself in a sermon at Friday prayers last week.

He said that today there is total freedom of speech in Iran.

There were people who did not believe in the system in the government or in the leader himself but they were allowed to speak, he said.

Coming on the eve of the general elections, the closures will inevitably strengthen fears that if the conservatives win the poll, the judiciary will feel it has a freer hand to continue its campaign against reformist publications.

The ballot boxes of Friday are the coffins of freedom. We will not take part in the funeral of freedom

Widely-circulated mobile phone text message

It is not clear how long the ban will last, but around 100 newspapers have been closed down in the past four years.

Many journalists and publishers have also been jailed.

Paris-based rights group Reporters Without Borders said last year that Iran had more journalists behind bars than any other country in the Middle East.

Hi-tech dissent

Some of Iran's best-known journalists and intellectuals have called for a boycott of the polls.

More than 100 journalists put their names to a statement saying the election will be neither free nor fair.

The BBC's Sadeq Saba reports many young people from the reformist camp are using mobile phone text messaging to advocate a boycott of the polls.

Diary-style internet sites - known colloquially as blogs - have also been fiercely critical of the upcoming election.

"It will be the end of the blog era," writes one online diarist, fearing a conservative victory on Friday will spell doom for the practice - widespread amongst young Iranians - of using the anonymity of the web to air diatribes against the clergy.